Hydraulic systems for driving saw mill carriages have been in use for some years. One such system satisfactory for the purpose is the Tyrone-Berry carriage Model No. SMA-260 manufactured by the Forest Products Division of Tyrone Hydraulics, Inc. of Ashland, Mississippi. This system utilizes fixed displacement hydraulic pumps and motors. The hydraulic system gives great speed and power plus instant reversibility and fast carriage return. There is a positive, fingertip control over carriage speed and direction which eliminates sawyer fatigue. The hydraulic pumps and motors are extremely durable and are ideally suited for the rugged service to which they are exposed in this type of operation. The drive is extremely smooth and flexible.
Some idea of the loads imposed on this type of equipment can be seen when it is realized that a saw mill carriage is frequently loaded with a log weighing a ton or more and desirably must accelerate this load very rapidly to a typical speed of 13 feet per second in about 4 feet. At the end of the stroke the carriage must be stopped rapidly and returned to initial position in the shortest possible time. The problem with such systems is that a large amount of energy is required for relatively short periods of time whereas for significant portions of a cycle of operation a relatively small fraction of rated horsepower is required. In known prior art systems, all of the energy required has been supplied by the prime mover of the carriage drive. With these systems, a relatively large prime mover is required, having a horsepower rating which is large enough to supply the peak horsepower requirements of the carriage. The problem with such a system is that such a prime mover is considerably larger than is required for most portions of the operating cycle and as a consequence is more expensive in the first instance and not as fuel efficient as is desirable.